The present invention relates to a voice coding communication system and an apparatus therefor and, more particularly, to a voice coding communication system which employs a 16 kbit/s voice coding system utilizing a low-delay code excited linear predition (hereinafter referred to as LD-CELP) scheme and an apparatus for implementing the voice coding communication system.
It is said that the voice activity factor in voice communications is around 35%.
With the recent diversification and internationalization of social and economic activities, expectations for mobile communication are rapidly rising. Among others, portable type car telephones and cordless telephones are now in highly increasing demand. Such portable terminals of mobile communication systems use batteries for the convenience of portability and the batteries need to stand long-time use; hence, the reduction of circuit power dissipation is required.
One method that has been proposed to reduce the circuit power consumption is a method which, noting the voice activity factor, actuates the transmitting circuit for the voice-active duration only and keep the circuit inoperative for the silent or voice-nonactive duration. This could be implemented by providing at the transmitting side a voice activity detector for detecting the voice activity and a discontinuous transmitter for stopping the operation of the transmitting circuit during the silent period.
In this instance, the receiving side presents a problem. That is, at the receiving side, the reproduced voice is discontinuous, and hence is very annoying. As is well-known in the art, this is attributable to the fact that during the transmission of a voice a background noise is superimposed on the voice but during the voice-nonactive or silent period no background noise is sent either; namely, the fact that the background noise is step-modulated according to the presence or absence of the voice signal.
A known solution to this problem is a method which generates, at the receiving side, a comfort noise similar to the background signal in the transmitting side while no voice signal is transmitted therefrom. This technique was studied first for a digital communication using a high efficiency voice coding system (13 kbit/s or lower) based on an analysis-synthesis scheme which analyzes and sends a voice signal and synthesizes it at the receiving side, and the technique has become widely known after establishment of its standardization algorithm for the digital car telephone.
On the other hand, the inventors of this application have already proposed a method for adding such capability to a 32 kbit/s adaptive differential PCM (ADPCM: Adaptive Differential Pulse Code Modulation) which is one of waveform coding techniques adopted as a standard voice coding system of a digital cordless telephone system (a full-rate system) (see Japanese Pat. Appln. No. 69747/92).
A half-rate system is now under development which will in the near future enable the digital cordless telephone system to increase the number of channels twice that in the full-rate system without changing the transmission rate of the radio section so as to implement efficient frequency utilization as in the case of the digital car telephone system. The standard voice coding system that is adopted in this half-rate system is a 16 kbit/s voice coding system employing the low-delay code excited linear prediction (LD-CELP) scheme (see TTC JT-G728 Standard).
As matters now stand, no study is being given on the technique of using, in connection with the LD-CELP scheme which is a half-rate standard voice coding system of the digital cordless telephone, the method of detecting a voice during the voice coding at the transmitting side and the capability of generating a comfort noise at the receiving and decoding side.